New possibilities: Floating tunnels
Traditional immersed tunnelling results in a tunnel buried beneath the waterway which it traverses. A new development- the submerged floating tunnel - consists of suspending a tunnel within the waterway, either by tethering a buoyant tunnel section to the bed of the waterway, or by suspending a heavier-than-water tunnel section from pontoons.
This technique has not yet been realised, but one project, in Norway, is currently in the design phase. The submerged floating tunnel allows construction of a tunnel with a shallow alignment in extremely deep water, where alternatives are technically difficult or prohibitively expensive.
Likely applications include fjords, deep, narrow sea channels, and deep lakes.